The present invention is directed to heat sinks primarily for use in dissipating waste heat generated by electrical and/or electronic components and assemblies. These heat sinks include a heat spreader plate and an assembly of heat conducting fins and reticulated foam structures that are bonded together. The foam block may be a continuous single block within the space between two adjacent fins along the length of the fins or may be an array of short-length blocks having intervening gaps along the length of the fins. Electronic components are connected to one surface of the spreader plate with the assembly of fins and foam connected to another surface of the spreader plate in contact with a cooling fluid.
High power electrical and electronic components continue to have an increasing demand for higher power dissipation within a relatively confined space. In order to provide for such higher power dissipation requirements while remaining suitably compact, several levels of thermal management are usually required at the device, sub-assembly and component level.
At the component level, various types of heat exchangers and heat sinks have been used that apply natural or forced convection or other cooling methods. A typical heat sink for electrical or electronic components is depicted in FIG. 1. As shown, the heat sink 10 includes a heat spreader plate 14 to which metal fins 12 are attached. An electronic component is attached to spreader plate 14 and a cooling fluid 16, such as air or water, is passed across fins 12 to dissipate the heat generated by the electronic component. For a given power level to be dissipated, the spreader plate size (i.e., area) and the fin length along the length of the cooling flow path can be calculated using known methods. Fin spacing and fin height are usually determined by known methods such as numerical modeling.
In heat sinks for low power density microelectronics such as computer chips, the chip is bonded directly onto the heat sink via an interface. More advanced heat sinks for high power density computer chips employ a separate spreader plate onto which is bonded the chip via an interface. The spreader plate/chip assembly is then bonded onto the heat sink via another interface. The presence of two interfaces significantly increases the overall thermal resistance to heat transfer from the chip to the coolant flow.
In demanding applications, the method of heat exchange is usually forced convection to the cooling fluid. In such systems, heat exchange can be improved by increasing the fin surface area exposed to the cooling fluid. This is accomplished by increasing the number of the fins per unit volume. However, there are limitations to achievable fin densities based upon manufacturing constraints and cooling fluid flow requirements.
Reticulated foams are also known in the art for their ability to conduct heat such as the metal foams disclosed in U.S. Pat. Nos. 3,616,841 and 3,946,039 to Walz, and the ceramic foams disclosed in U.S. Pat. No. 4,808,558 to Park et al. Metal foams have been sold under the trade name DUOCEL available from Energy Research and Generation, Inc., Oakland, Calif.
Until recently, metal and ceramic reticulated foams have not been adapted for use in heat sinks for dissipating waste heat from electronic components. However, these structures, especially when comprised of metal, make excellent heat exchangers because of their conductivity and their extremely high surface area to volume ratio. While earlier porous heat exchangers had up to 100 open cells per square inch, reticulated foam has up to 15,625 open cells per square inch. Reticulated foam is far more porous and has far more surface area per unit volume (1600 square feet/cubic foot) than heat exchangers having other structures. The pressure drop of fluids flowing through reticulated foam is also relatively low so that movement of a cooling fluid through the foam is practical.
Studies by Bastawros have now shown the efficacy of metallic foams in forced convection heat removal for cooling of electronics. See, Bastawros, A. -F., 1998, Effectiveness of Open-Cell Metallic Foams for High Power Electronic Cooling, ASME Conf. Proc. HTD-361-3/PID-3, 211-217, and Bastawros, A. -F., Evans, A. G. and Stone, H. A., 1998, Evaluation of Cellular Metal Heat Transfer Media, Harvard University report MECH 325, Cambridge, Mass. Bastawros demonstrated that the use of metallic foam improved heat removal rate with a moderate increase in the pressure drop. Bastawros"" results were based on thermal and hydraulic measurements (on an open cell aluminum alloy foam having a pore size of 30 pores per inch) used in conjunction with a model based upon a bank of cylinders in cross-flow to understand the effect of various foam morphologies. The model prediction was extrapolated to examine the trade-off between heat removal and pressure drop. The measurements showed that a high performance cellular aluminum heat sink (i.e., aluminum foam) removed 2-3 times the usual heat flux removed by a pin-fin array with only a moderate increase in pressure drop.
A range of new heat sinks for electrical and electronic components is herein presented that provides for space-efficient heat exchange with low thermal resistance. These heat sinks are capable of removing the increased waste heat flux generated by today""s higher power electronic systems.
In general, heat sinks of the present invention comprise a spreader plate, at least two fins and at least one porous reticulated foam block that fills the space between the fins. The foam block may be a continuous single block within the space between two adjacent fins along the length of the fins or may be an array of short-length blocks having intervening gaps in-between the blocks along the length of the fins. All materials are made from a heat conducting material. The fins and foam blocks form an assembly that is connected to one surface of the spreader plate. Electronic components to be cooled are preferably connected to an opposing surface of the spreader plate, but may be connected to any surface of the spreader plate suited for heat transfer.
In the present invention, the preferred dimensional length is defined for the foam blocks used, whether they comprise continuous single foam blocks (hereinafter referred to as xe2x80x9ccontinuous blockxe2x80x9d) or an array of short-length foam blocks (hereinafter referred to as xe2x80x9cinterrupted blockxe2x80x9d) along each fin length. The present invention also defines a relationship for determining the optimum interrupted block length when such an array is selected. The present invention further defines the preferred dimensional relationships for establishing the optimum fin spacing and fin height for the heat sinks provided herein. Devices produced using these dimensional relationships find particular use in cooling microelectronic components such as microprocessors.